Tuning element for broadcast receiver sets



May 23, 1939. Q WOHLFARTH 2,159,754

TUNING ELEMENT FO R BROADCAST RECEIVER SETS Filed Jan. 2, 1956 INVENTOR.

0. WOHLFARTH 7 ATTORNEY.

Patented May 23, 1939 PATENT OFFICE TUNING ELEMENT FOR BROADCAST RECEIVER- SETS Otto Wohlfarth, Berlin, Germany,

Allgemeine Elektricitats Gesellschaft,

assignor to Berlin,

Germany, a corporation of Germany Application January 2, 1936,

In Germany March 4,

2 Claims.

tocover an entire wave band, say, from 500 to 1500 kc., it is necessary that the core should be shifted to such an extent that in one extreme position it will be located inside the densest coil field, while in the opposite extreme it will be n placed as far as feasible outside the coil field.

The variometer arrangements of this nature heretofore known in the art involve the disadvantage that the core must be displaced in the direction of one of its axes, usually the longitudinal axis so that a complicated gear mech- .anism is needed in order that such shifts may be changed into a rotary motion usual in broadcast receiver sets furnished with knobs, or vice versa. This is especially true if the actuation of the variometer is to be mechanically combined with the actuation of some other part in the receiver set, for example, the rotary condenser.

It has for this reason not been attempted in the prior art to couple a rotary condenser and an iron-cored variometer mechanically. Now, the present invention discloses ways and means whereby such a mechanical coupling is accomplished without gears of any kind.

On the other hand, variometers have been disclosed in the prior art in which changes in inductance are effected by variation of permeability rather than the shifting of an iron-core. The said change in permeability is obtained by varying the biasing magnetization of the iron coils. For instance, an auxiliary winding wrapped on the iron-core could be connected with a source of 3 voltage existing in the set, the current caused to fiow through the said auxiliary winding being regulated by a variable resistance or potentiometer.

According to the present invention, the said regulating means is so designed that variation of permeability is insured by a rotary movement. The basic idea of the invention is carried into effect, for example, in such a way that a variable resistance is concentrically mounted with.

the shaft of the rotary condenser or condensers.

For a better understanding of the invention reference is made to the accompanying drawing showing a single embodiment thereof with certain Serial No. 57,172

circuits schematically shown. In the drawing the condenser shaft carrying rotor sections 8, 8 has a control knob ll secured to one end and a rotatable contact arm l2 to its other, the contact arm engaging a wire wound resistor 1 concentric with shaft Ill. The rotor sections 8, 8' cooperate with stator sections 9, 9 to provide tuning condensers which are connected to the fixed coils l, I. By making coils l and I of different sizes, coil l and its condenser may be designed to form a resonant circuit tunable over one frequency range while coil I and its condenser form a resonant circuit tunable over a different frequency range. The coils I and l are mounted on iron cores 2 and 2 which are provided with magnetizing windings 3, 3'. Each of these windings is connected as shown in series with a battery or other steady source of potential 4 and the variable resistor I so that the magnetizing currents through these windings are controlled simultaneously with the adjustment of the rotary condensers. The inductance values of coils l, I will be correspondingly changed and the resonant frequencies of the two circuits may by this means be adjusted to any desired values.

The invention ofiers particular advantages in the case of multi-circuit receiver sets inasmuch as the variation of the inductance of several coils requires only one variable resistance. Moreover, in contrast with the arrangements of the prior art an appreciable simplification is obtainable in receivers covering several wave bands since one and the same regulating means is usable jointly for several wave bands.

Having described my invention, what I claim and desire to secure by Letters Patent is:

1. In combination, a circuit comprising a coil having a core of magnetic material, a condenser connected to said coil, means for varying the capacity of said condenser and the magnetic flux density of said core to tune said circuit over a band of frequencies, a second circuit comprising a coil having a core of magnetic material, a condenser connected to said second named coil, means for varying the capacity of said second condenser and the magnetic flux density of said second named core to tune said second named circuit over a different band of frequenciesand a single control means arranged to operate both said capacity varying means. 7

2. In combination, a first coil having a core of magnetic material, a second coil of different inductance than said first coil and. having a core of magnetic material, a magnetizing winding for each of said cores, a direct current circuit connected to said windings and including a single resistor, a first condenser connected across the terminals of said first coil and having adjustable plates, a second condenser connected across said second coil and having adjustable plates, a single control means for moving the adjustable plates of both said condensers and a contact connected to said control means and adjustable along said resistor whereby said coils are tuned through difierent ranges of frequencies by adjustment of said control means.

o'r'ro WOHLFARTH. 

